1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and more specifically to a metal gate electrode and its method of fabrication.
2. Discussion of Related Art
A conventional MOS transistor is shown in FIG. 1. On a substrate 110, usually silicon, rests a thin gate dielectric layer 112, usually made of, but not limited to silicon dioxide. Upon the thin gate dielectric layer 112, is a gate electrode 120, an electrically conductive material. Together the thin gate dielectric layer 112 and the gate electrode form a gate structure 122. Adjacent to the gate structure 122 are spacers 130, made of a dielectric material. The spacers 130 are aligned directly over shallow junctions 128 of source 124 and drain 126. The source 124 and drain 126 have deeper regions 132 and 134, over which lays a silicide 136, which subsequently can be coupled to metal interconnect lines that run throughout the integrated circuit. Spacers 130 separate the gate structure from the silicide to prevent silicide formation on walls of the gate electrode 120.
When the transistor is in use, the gate structure is electrically charged and a channel region 138 forms beneath the gate allowing current to flow from the source to the drain. Thus, the gate electrode 120 must be an electrically conductive material. Doped polysilicon is the material of choice. In conventional methods of MOS transistor fabrication, the gate structure is formed before the source and drain regions are doped to act as a protective mask to the channel region. The doped polysilicon will prevent the dopants from reacting with the channel region of the underlying substrate. When a charge of the correct polarity is applied to the electrode, the channel region electrically inverts and becomes a conductive path between the source and drain regions. However, polysilicon has its drawbacks. One drawback is polydepletion, or voltage leakage. Another drawback is that polysilicon is highly resistive and therefore presents current flow-problems.
Metal is another material used for the gate electrode. Metal has various advantages over polysilicon as a gate electrode material. For instance, metal allows for excellent current flow and metal has less voltage depletion problems than polysilicon. However, metal too has its drawbacks. Some metals, such as Ti and Ni, are highly diffusive and act as contaminants within the channel region, particularly during the high temperature conditions required for dopant activation of the source/drain implant. Also, certain work functions are required that allow MOS transistors to work optimally, and it is more difficult to manipulate the work function of metals than it is to manipulate the work function of polysilicon. Furthermore, metals are difficult to etch properly. Dry-etch methods are too harsh on underlying Si substrate while wet-etch methods can excessively undercut the sidewalls of the gate electrode.
Some recent methods have attempted to solve some of these problems by combining the conventional methods of forming the transistor, with polysilicon as the gate electrode during doping, with the additional steps of completely etching out the polysilicon after doping and replacing it with a metal. However, this replacement process is complex and can easily result in costly errors if not done correctly. Therefore, it would be advantageous to have a process of making a metal gate electrode, but without the complexity of the current replacement process.